The physical properties of minerals: hardness, density, streak and transparency

Identifying a mineral does not rely solely on its appearance. The physical properties of minerals provide reliable criteria for observing, comparing and distinguishing specimens. Hardness, density, streak and transparency allow for more rigorous identification, beyond simple colour variations.

In this article, we examine these key properties and explain how they can be used in practice.

What is mineral hardness? (Mohs scale)

Hardness refers to a mineral’s resistance to scratching. It is measured using the Mohs scale, developed in 1812 by the German mineralogist Friedrich Mohs. This scale ranks minerals from 1 to 10.

Practical use

Knowing the hardness of a mineral helps assess its resistance to wear and scratching. The Mohs scale provides a simple and practical reference for comparing minerals.

Common reference points:

• Talc and gypsum (≈ 1–2): can be scratched with a fingernail.
• Calcite (≈ 3): can be scratched with copper.
• Fluorite (≈ 4) and apatite (≈ 5): can be scratched with a steel blade.
• Feldspar, quartz, topaz, corundum and diamond: hard enough to scratch glass (depending on conditions and glass hardness).

Mineral density: definition and calculation

Density indicates how heavy a mineral is relative to its volume. It is the ratio between the weight of the mineral and the weight of an equal volume of water. For example, a density of 3 means the mineral is three times heavier than the same volume of water.

Density is a precise identification criterion when weight and volume are measured accurately. In practice, it is often used alongside other properties such as hardness, lustre and streak.

How to calculate density

Required equipment:

• A precision scale (to measure weight).
• A thin wire or support (to suspend the mineral).
• A container of water (for immersion).

Method:

1. Weigh the mineral and record its weight as P_real.
2. Place the container of water on the scale and tare it (reset to zero).
3. Suspend the mineral and fully immerse it in the water without touching the container walls.
4. Measure the apparent weight in water and record it as P_water.

Formula:

Density =
P_real
──────────────
P_real − P_water

Streak: a reliable identification criterion

Streak refers to the colour of the fine powder left when a mineral is rubbed on an unglazed porcelain plate. This test is often more reliable than observing surface colour, which may vary due to impurities.

Examples

• Hematite: regardless of external appearance (red, black or metallic), its streak is reddish-brown.
• Pyrite: produces a greenish-black streak.
• Calcite: leaves a white streak.

Streak helps distinguish minerals that appear similar. For example, pyrite and gold may look alike, but their streak differs: gold leaves a yellow streak, while pyrite leaves a darker streak.

Mineral transparency: definition and distinctions

Transparency describes how light passes through a mineral and is an important observational criterion.

1) Transparent

A mineral is transparent if light passes through clearly and objects can be seen distinctly.

Examples: rock crystal (quartz), diamond.

2) Translucent

A mineral is translucent if light passes through but objects cannot be seen clearly; light is diffused.

Examples: calcite, fluorite, some varieties of quartz.

3) Opaque

A mineral is opaque if no light passes through it.

Examples: hematite, pyrite, galena.

Conclusion

Hardness, density, streak and transparency are essential physical properties for mineral identification. Combined with other criteria such as lustre, cleavage and crystal structure, they allow for more accurate observation and comparison.

Regular observation and hands-on practice help develop more confident identification skills. To explore more specimens, you may browse our catalogue:

Browse our available minerals